Piezo ceramic actuators are used in a broad range of applications requiring micron scale movements with sub-micron resolution. The movement is made possible by applying a voltage across the actuator material causing the lattice to distort. The direction of the distortion is orientated to result in an extension of the material. The extension capabilities can be enhanced by building up the actuator with multiple, alternate layers of piezo material and electrode.
The extension characteristics of piezo actuators have a high degree of hysteresis and a non-linearity that alter the scale of the hysteresis loop over the range of cyclic applied voltages. The devices will also creep to their final extension. In applications requiring knowledge of position there is a need for accurate measurement of actuator extension.
An established method of making accurate position measurements uses a capacitor sensor. One plate of the capacitor is attached as a reference surface while the other moves with the piezo actuator. A change in gap between the two plates alters the capacitance. As the changes in gap are small, the sensing capacitor is frequently arranged as one arm of a bridge circuit and an AC excitation is used to remove drifts associated with DC measurements.
A problem with capacitor sensors is that complications can arise in dealing with fringing fields round the capacitor plates, which increase the ‘apparent’ plate area of the capacitor and distort the measurement. In addition, stray capacitances in the associated circuitry can lead to additional distortions, as can electromagnetic interference.